A CMM is a device which has three mutually perpendicular axes, each one having attached to it a measurement scale (a linear electronic measuring device) and counter. A counter is an electronic device that keeps track of the output of a scale and converts the electronic pulses into a digital output. When a signal is received, the counter will "latch", that is, the counter will store the exact value it had when the signal was received in a buffer, but continue to keep track of the machines current position as it continues to move.
The axes on the CMM are commonly known as X which measures left to right, Y which measures front to back, and Z which measures up and down. The part of the CMM that moves in the Z direction is called the Z-rail. A stylus is attached either directly, or through a probe to the bottom of the Z-rail. The stylus is a shaft with a ball on one end and a threaded shank on the opposite end for securing to the frame. The axes of the CMM are arranged in such a way that movement of any one, two, or all three of them result in movement of the stylus. When this stylus comes in contact with the workpiece, the three counters must be "latched" by a signal from either a hand switch or an electronic probe.
A CMM properly equipped with motors, drive shafts, computers, and the like, may have the stylus position under numerical control, however virtually all machines on the market today are equipped with frictionless air bearings and so an operator may position the stylus by grasping the Z-rail and with it position the three axes as required. Such a CMM is known to be manually controlled.
An electronic probe is a device, for use on a CMM, which has a flexible coupling consisting of a moveable and a fixed member, a means for attaching a stylus to the movable member of the flexible coupling, a means for detecting that the moveable member has been displaced and/or that the stylus has made contact with the workpiece and a means for signaling the counters to latch.
The accuracy of an electronic probe is determined by a number of factors as follows:
1. The ability of the moveable member of its flexible coupling to return to the exact point in space relative to the fixed member, after a force which displaced it is removed
2. The difference in the amount of displacement of the moveable member of the flexible coupling, in all directions, required for it to signal contact with the workpiece
3. The time required by the electronics, to signal the counters, multiplied by the variation in speed at which the stylus is traveling, when it makes various contacts with the workpiece.
A numerically controlled CMM must have its stylus attached to the Z-rail through an electronic probe as there is no operator to work a hand switch to latch the counters, and the flexible coupling incorporated in all known electronic probes is required to prevent damage to the stylus, probe, and/or CMM in the event that the Z-rail does not stop immediately upon contact with the workpiece.
It is common practice to have the stylus of a manually controlled CMM attached to the Z-rail without an electronic probe and flexible coupling. It is possible to do so as the operator is relied upon to reduce the speed of the stylus (and therefore the three axes of the machine) as it approaches the workpiece and his wrist and arm act in place of a flexible coupling and allow the stylus to "bounce" off the piece. It should be noted that the styli used in this fashion are considerably more rugged with shafts typically having a 3/8" diameter whereas the styli used with electronic probes have diameters less then 1/8". In fact the heavier styli are sometimes referred to as "solid probes" a term which has led to confusion in the past. When used in this manner, the operator brings the ball of the stylus into contact with the workpiece, holds it there until the machine "settles", and then depresses the hand switch to latch the counters. This method has the following problems:
1. Depending on where and with how much force the stylus is held in contact with the work piece, the stylus and or entire Z frame may be deflected.
2. The work piece is warmed by the operators hands while waiting for the machine to settle.
3. If a reading is taken before the machine is fully settled errors may occur.
4. If the switch is not in an acceptable location the probe may be moved while reaching for it.
5. The method is inherently slow.
6. The method is highly susceptible to variations due to operator's pressure of stylus on the work piece.